Diagnosis and classification of diabetes mellitus.

Hemoglobin A1c (HbA1c) is a marker of cumulative glycemic exposure over the preceding 2- to 3-month period. Whether mild elevations of this biomarker provide prognostic information for development of clinically evident type 2 diabetes and cardiovascular disease among individuals at usual risk for these disorders is uncertain. We examined baseline HbA1c levels as a predictor of incident clinical diabetes and cardiovascular disease (nonfatal myocardial infarction, coronary revascularization procedure, ischemic stroke, or death from cardiovascular causes) in a prospective cohort study beginning in 1992 of 26,563 US female health professionals aged 45 years or more without diagnosed diabetes or vascular disease (median follow-up 10.1 years). During follow-up, 1238 cases of diabetes and 684 cardiovascular events occurred. In age-adjusted analyses using quintiles of HbA1c, a risk gradient was observed for both incident diabetes and cardiovascular disease. After multivariable adjustment, HbA1c remained a strong predictor of diabetes but was no longer significantly associated with incident cardiovascular disease. In analyses of threshold effects, adjusted relative risks for incident diabetes in HbA1c categories of less than 5.0%, 5.0% to 5.4%, 5.5% to 5.9%, 6.0% to 6.4%, 6.5% to 6.9%, and 7.0% or more were 1.0, 2.9, 12.1, 29.3, 28.2, and 81.2, respectively. Risk associations persisted after additional adjustment for C-reactive protein and after excluding individuals developing diabetes within 2 and 5 years of follow-up. These prospective findings suggest that HbA1c levels are elevated well in advance of the clinical development of type 2 diabetes, supporting recent recommendations for lowering of diagnostic thresholds for glucose metabolic disorders. In contrast, the association of HbA1c with incident cardiovascular events is modest and largely attributable to coexistent traditional risk factors.

The incidence of self-reported diagnosed diabetes may be increasing because of recent changes in the diagnostic criteria for diabetes, enhanced case detection, and a true increase in disease incidence. These factors may also be changing the characteristics of newly diagnosed cases. Therefore, we examined recent trends in the incidence of diagnosed diabetes, changes to the characteristics of incident cases, and factors associated with incidence. First, National Health Interview Survey data for 1997 to 2003 were used to examine 7-year trends in the incidence of diagnosed diabetes among U.S. adults aged 18 to 79 years. Second, among 1997-1998 and 2002-2003 incident cases, differences in sociodemographic characteristics, risk factors, and indicators of health status were examined. Lastly, multivariate-adjusted incidence from multiple logistic regression of 2001-2003 survey data were derived. From 1997 to 2003, the incidence of diagnosed diabetes increased 41% from 4.9 to 6.9 per 1,000 population (p <0.01). Incidence increased among men and women, non-Hispanic whites, persons with at least a high school education, nonsmokers, active and inactive persons, and among obese persons (p <0.05). Obesity was more prevalent (p <0.01) and physical limitation was less prevalent (p =0.03) in 2002-2003 versus 1997-1998 incident cases. Multivariate-adjusted incidence increased with age and BMI category, and decreased with education level (p <0.05). Obesity was a major factor in the recent increase of newly diagnosed diabetes. Lifestyle interventions that reduce or prevent the prevalence of obesity among persons at risk for diabetes are needed to halt the increasing incidence of diabetes.